1. Field of the Invention
The present invention relates to a motor and more particularly to a spindle motor which drives recording disks.
2. Description of the Related Art
A spindle motor, as an example of a motor in general, includes a housing, a shaft member fixed in the housing, and a hub (which functions as a rotor) rotatably supported on the shaft member through a bearing means. A rotor magnet is provided on the inner circumferential surface of the hub and a yoke is provided facing with the rotor magnet. Thus, the recording disks such as magnetic disks can be mounted on the hub in a desired manner.
In the spindle motor of this kind, when the shaft member is made of stainless steel and the housing is made of aluminum, for example, the following shortcomings are produced. Namely, when the shaft member is fixed in the housing by a press fitting, the inner circumferential surface of the supporting hole of the housing for the shaft member will be damaged by the shaft with a high possibility due to the difference in hardness of the materials between the shaft member and the housing. When the inner circumferential surface of the supporting hole is damaged, a fitting accuracy between the shaft member and the housing will become worse, then the rotational accuracy of the hub will fall, which will cause a deflection of the recording disks.
In addition, there exists a sort of the spindle motor which has a hub rotatably supported through a pair of bearing means and has a bushing placed between the one part of the pair of bearing means and the yoke mounted on the inner circumferential surface of the hub. Generally, the hub is made of aluminum and the yoke member is made of iron. When the hub and the yoke are secured together by such means as a shrink fitting over the relatively long span in their axial direction, thermal stress can be generated due to the difference in thermal expansion between the hub and the yoke. In addition, when the bushing made of the same iron as the material of joke is inserted into the yoke by a press fitting in substantially overall span of the bushing, the thermal stress will be also generated between the yoke and the bushing member. Then, in fitting area between the bushing member and the yoke, a clearance will be created and this clearance may cause an eccentricity in the rotation center of the hub.
Furthermore, there also exists a sort of the spindle motor which has a hub rotatably supported through bearing means at both ends of the hub and has a magnetic fluid seal means provided on the outer area of the bearing means. In this type of the motor, by means of this magnetic fluid seal means, an interior area of the motor can be sealed almost completely from an exterior area to prevent lubricant particles on the interior area of the motor from dispersing into the exterior area of the motor. Accordingly, the magnetic fluid seal means have been preferably employed into a field of such as a hard disk unit, where a disk room outside the motor must be maintained in clean condition. However, when there is a pressure difference exceeding a certain limit between the interior and exterior areas of the motor caused by a variation of temperature or atmospheric pressure, a magnetic fluid of the magnetic fluid seal means could be dispersed becoming intolerable to the pressure difference.